Flow accelerated corrosion and erosion−corrosion behavior of marine carbon steel in natural seawater

In this work, flow accelerated corrosion (FAC) and erosion−corrosion of marine carbon steel in natural seawater were electrochemically studied using a submerged impingement jet system. Results show that the formation of a relatively compact rust layer in flowing natural seawater would lead to the FAC pattern change from ‘flow marks’ to pits. The increase of the flow velocity was found to have a negligible influence on the FAC rate at velocities of 5−8 m s−1. The synergy of mechanical erosion and electrochemical corrosion is the main contributor to the total steel loss under erosion−corrosion. The increase of the sand impact energy could induce the pitting damage and accelerate the steel degradation. The accumulation of the rust inside the pits could facilitate the longitudinal growth of the pits, however, the accumulated rusts retard the erosion of the pit bottom. The erosion and corrosion could work together to cause the steel peeling at the pit boundary. The steel degradation would gradually change from corrosion-dominated to erosion-dominated along with the impact energy increasing.

Prediction of corrosion rates of a ship under the flow accelerated corrosion mechanism

In order to reveal the corrosion of a ship hull in flowing seawater, which is difficult to study under experimental conditions, a numerical method was adopted. Firstly, the numerical model of seawater flow field around the hull was established and computed, and the hydraulic characteristics of the flow field such as velocity, pressure, and oxygen concentration distribution were got. The distribution of oxygen has a great influence on corrosion. Generally speaking, where the oxygen concentration is higher, the corrosion rate is higher. The results show that the oxygen concentration is higher at the rear of the hull bottom surface. Next, a corrosion model was founded based on the above calculation. Through the calculation of the corrosion model, the mass transfer coefficient and corrosion rate distribution on the hull surface were obtained and the final conclusion was got flow accelerated corrosion (FAC) mechanism was revealed. The conclusion is as follows: Comparatively speaking, the corrosion rate of the middle and rear part of the bottom and the middle part of the sideboard is higher. Generally speaking, the corrosion rate increases with the increase of flow rate.